Maltol is a naturally occurring substance found in the bark of young larch trees, pine needles and chicory. Maltol has the structure ##STR1## Maltol is one of a family of compounds called gamma pyrones which enhance the flavor and aroma of a variety of food products and which, in general, have the formula ##STR2## wherein R is hydrogen, (C.sub.1 -C.sub.6)alkyl, (C.sub.2 -C.sub.6)alkenyl, phenyl, or benzyl. The compounds wherein R is methyl (maltol) or ethyl (ethyl maltol) are regulated for use as flavor enhancers in foods. In addition, these materials are used as ingredients in perfumes and essences. Pyromeconic acid (formula II, R.dbd.H) derivatives such as the 2-alkenylpyromeconic acids reported in U.S. Pat. No. 3,365,469 inhibit the growth of bacteria and fungi and are useful as flavor and aroma enhancers in foods and beverages and aroma enhancers in perfumes.
Many processes for producing maltol have been reported. Early commercial production was from the destructive distillation of wood. Synthesis of maltol from 3-hydroxy-2-(1-piperidylmethyl)-1,4-pyrone was reported by Spielman and Freifelder in J. Am. Chem. Soc., 69, 2908 (1947). Schenck and Spielman, J. Am. Chem. Soc., 67, 2276 (1945), obtained maltol by alkaline hydrolysis of streptomycin salts. Chawla and McGonigal, J. Org. Chem., 39, 3281 (1974) and Lichtenthaler and Heidel, Angew. Chem., 81, 998 (1969), reported the synthesis of maltol from protected carbohydrate derivatives. Shono and Matsumura, Tetrahedron Letters No. 17, 1363 (1976), described a five step synthesis of maltol starting with methyl furfuryl alcohol.
Syntheses of gamma-pyrones such as pyromeconic acid, maltol, ethyl maltol and other 2-substituted-3-hydroxy-gamma-pyrones are described in U.S. Pat. Nos. 3,130,204; 3,133,089; 3,140,239; 3,159,652; 3,365,469; 3,376,317; 3,468,915; 3,440,183; 3,446,629; 4,082,717; 4,147,705; 4,323,506; 4,342,697; 4,387,235; 4,390,709; 4,435,584; and 4,451,661. A typical process for making gamma pyrones is illustrated by U.S. Pat. No. 4,435,584 to Brennan et al. This patent describes a one pot, furfuryl alcohol based synthesis wherein all reactions are conducted in aqueous/protic media. The present invention is based on chemistry which can employ a furfuryl alcohol precursor, but which, by contrast, employs a reaction medium comprising an aprotic solvent to effect the conversion of a haloenone to the desired gamma pyrone product.
U.S. Pat. No. 4,126,624 discloses preparation of gamma-pyrones from 3-substituted furans by procedures which can employ isolation of various components by extraction with various organic solvents.